The present invention relates to the filtering of a gas stream for the removal of particulate matter entrained therein, and more particularly, to a method for cleaning the fabric filters housed in the various filter chambers of a multiple chamber fabric filter apparatus.
One highly successful technique for separating undesirable particulate matter from a gas stream laden with particulate matter is fabric filtration. Such filtration is typically carried out in a fabric filter apparatus, commonly referred to as a baghouse, which houses fabric filter means upon which at least a portion of the particulate matter in the gas stream traverses the filter means. Typically, the baghouse is comprised of a plurality of independent, isolatable filter chambers each of which houses a plurality of fabric filter bags suspended from a skeletal support system. The particulate matter-laden gas stream enters the baghouse from one end and, under the influence of a fan either disposed upstream or downstream of the baghouse, passes through the filter bags and out the opposite end of the baghouse, with the particulate matter being retained on the upstream side of the filter bags which the gas must traverse as it flows through the baghouse. The gas that is discharged from the baghouse is a relatively particulate matter-free gas stream and vented to the atmosphere.
Continuous operation of such a filter apparatus causes a cake of particulate matter to build up on the upstream surface of the filter bags or other fabric filter means. As this cake of dust builds up on the surface of the filter means, the pressure drop across the filter means increases whereupon it ultimately becomes necessary to effect removable of the filter cake so as not to exceed the pressure-volume capabilities of the flow of the inducing fan. Therefore, it has become customary to effect removal of the particulate matter cake building up on the filter means on a periodic basis when the pressure differential or drop across the baghouse has reached a preselected upper limit of desired operating-pressure differential. Typically, when the gas pressure drop between the inlet flange and the outlet flange of the collector has reached this upper limit, the cleaning is initiated by isolating one of the chambers of the collector and cleaning that chamber, then sequentially isolating and cleaning each of the remaining chambers until all chambers of the dust collector have been cleaned in fairly rapid succession. Alternately, the cleaning process may be terminated at a predesignated lower limit value on pressure drop across the dust collector even though all of the individual chambers may not have been cleaned in that particular cleaning sequence.
In either case, there is a significant change in pressure drop across the dust collector from the initiation of the cleaning process to termination of the cleaning process. The cleaning process is not reinitiated until the pressure drop across the dust collector has again reached the upper limit value. Therefore, in normal operation of the dust collector, the pressure differential across the dust collector will vary significantly from clean to dirty operation.
Although such a control system has proven satisfactory for processes where operational conditions, i.e. gas flow and dust concentration, are substantially constant, such a system is not well suited for use in operations where gas flow or dust concentration may vary significantly. For instance, when such a fabric filter dust collector is utilized to clean the flue gas from a furnace, the dust collector will see significantly varying gas flows and dust concentrations depending upon the nature of the fuel being burned and the load at which the furnace is operating at any given time. Under conditions of high gas flow or high dust concentrations, the filter cake will build up more rapidly on the bags causing the pressure differential across the dust collectors to increase rapidly. Conversely, under conditions of low gas flow or low dust concentration, the filter cake will build up more slowly and consequently the pressure differential across the dust collector will increase more slowly. Obviously, the more rapidly the pressure differential across the dust collector builds up, the more frequent the cleaning cycle must be initiated. Thus, in the type of system described above wherein there is a significant difference in pressure differential across the dust collector between the clean mode and the dirty mode, there will be a very erratic cleaning frequency profile depending upon the dust concentration and the gas flow history. An erratic cleaning frequency only complicates operation of the dust collector equipment and may lead to an imbalance in gas flow and dust load amongst the various chambers of the dust collector.